1. Field of the Disclosure
The disclosure relates to a multi-core optical fiber and a method of optical transmission.
2. Description of the Related Art
As one of techniques allowing large capacity transmission, mode division multiplexing (MDM) transmission has attracted attention (see Non Patent Literature 1: M. Salsi et al., “Transmission at 2×100 Gb/s, over Two Modes of 40 km-long Prototype Few-Mode Fiber, using LCOS based Mode Multiplexer and Demultiplexer,” OFC2011, PDPB9; for example). This mode division multiplexing transmission is a technique of propagating a plurality of signal light beams respectively in a plurality of different propagation modes of an optical fiber. By this mode division multiplexing transmission, a transmission capacity per one optical fiber can be increased largely. In conventional mode division multiplexing transmission, as an optical fiber to be a transmission medium, a multi-mode optical fiber having a plurality of higher-order propagation modes is presumed to be used. Particularly, few-mode fibers for which each transmission mode is easily controllable have attracted attention. There is a possibility that interference among signal light beams respectively propagating in their modes during their propagation causes distortion of signal light and thus deterioration in signal quality. By using a multiple-input multiple-output (MIMO) technique utilized in the wireless field, studies for ensuring signal quality have been performed (see Non Patent Literature 1 and Non Patent Literature 2: R. Ryf et al., “Space-division multiplexing over 10 km of three-mode fiber using coherent 6×6 MIMO processing,” OFC2011, PDPB10; for example). This MIMO technique utilized in the wireless field is a technique of transmitting different independent signals on the same channel from a plurality of transmission antennas at a transmitting station end, receiving the signals by using a plurality of antennas similarly at a receiving station end, obtaining a transfer function matrix between each transmission antenna and the receiving antennas, estimating the independent signal transmitted from each antenna at the transmitting station end by using the transfer function matrix, and reproducing data. In optical fiber transmission also, by obtaining a transfer function matrix of an optical fiber to be a transmission medium using training signals, independent signal light beams transmitted from a transmitting station end is able to be estimated.
As means for also realizing large capacity transmission, multi-core optical fibers each having a plurality of core portions in one optical fiber have attracted attention. It is expected that by propagating a different signal light beam in each core portion of the multi-core optical fiber, large capacity by space division multiplexing (SDM) transmission is achievable. Recently, experimental results of space division multiplexing transmission using multi-core optical fibers have started to be reported (see Non Patent Literature 3: B. Zhu et al., “Space-, Wavelength-, Polarization-Division Multiplexed Transmission of 56-Tb/s over a 76.8-km Seven-Core Fiber,” OFC2011, PDPB7 and Non Patent Literature 4: B. Zhu et al., “112-Tb/s Space-division multiplexed DWDM transmission with 14-b/s/Hz aggregate spectral efficiency over a 76.8-km seven-core fiber,” 15 Aug. 2011/Vol. 19, No. 17/OPTICS EXPRESS pp. 16665-16671, for example).
To realize mode division multiplexing transmission, a plurality of signal light beams need to be simultaneously input to an optical fiber, which is a transmission medium, and coupled to respective propagation modes. In a higher-order propagation mode, a field distribution of light is asymmetrical with respect to a propagation axis of the optical fiber and hence, in order to be effectively coupled to each propagation mode, each signal light beam must be input with its input position at a facet of the optical fiber being adjusted. As such input means, a spatial coupling system or the like using a liquid-crystal-on-silicon (LCOS) element (Non Patent Literature 1) or a beam splitter element (Non Patent Literature 2) has been proposed.
However, both of these elements require complex structures and precise adjustments. Further, in spatial coupling systems, improving coupling efficiency or reduction in optical loss is difficult.
Accordingly, there is a need to provide a multi-core optical fiber for mode division multiplexing transmission and a method of optical transmission that do not require complex structures or precise adjustments when a plurality of signal light beams are input simultaneously and that are able to reduce optical loss.